Investigators said today they probably will never be able to tell from pathological tests whether champion golfer Payne Stewart and five others died of oxygen deprivation before their Learjet nosedived into a cow pasture in this remote corner of northeastern South Dakota on Monday.

Federal safety officials also said a voice recorder inside the cockpit of the doomed plane probably will be useless because it likely only has a recording of the final 30 minutes of the flight, by which time there was no sign of life on board.

Crash investigators began painstakingly sifting through the wreckage of the Learjet that carried Stewart and the others to their deaths after a ghostly four-hour flight with the pilots and passengers apparently dead from suspected depressurization--or decompression--of the cabin.

In 1995, the Federal Aviation Administration directed owners of several Learjet models, including the Learjet 35, to replace valves that regulate cabin pressure because of a "possible failure . . . which could result in rapid decompression of the airplane." The directive, which gave the aircraft owners 18 months to comply, also prohibited pilots from flying the plane above 41,000 feet until new valves were installed.

The Learjet 35 that crashed has been operated since January by Sunjet Aviation of Sanford, Fla. James Watkins, the company's president, said in a telephone interview today that the aircraft's maintenance log books showed that the new valves had been installed.

In addition, Watkins said that when Sunjet began operating the plane it underwent a mandatory FAA inspection and was certified to be in compliance with all FAA directives that applied to Learjet 35s.

There was little left of the executive jet when six investigators of the National Transportation Safety Board went to the crash scene here, 15 miles southwest of Aberdeen, except for a crater about 30 feet wide and 10 feet deep filled with tangled and fused metal. Smaller bits of the aircraft were scattered over an area about half the size of a football field.

Robert Francis, vice chairman of the NTSB, said initial evidence pointed to some sort of explosive decompression after the Learjet took off from Orlando on Monday, but only a complex investigation that could take months would rule out other possible causes.

Shortly after the plane took off Monday morning on a flight plan that called for an altitude of 39,000 feet, the aircraft inexplicably climbed in a rapid ascent to 41,000 feet. It veered off its intended course to Dallas, on a 1,400-mile path across the nation, shadowed by Air Force F-16 fighters whose pilots were helpless to do anything but watch the aircraft eventually spiral earthward and crash.

Francis said that because the wreckage was "very concentrated in the primary impact area," finding clues to the cause of the apparent decompression may be difficult. He said there was no flight recorder--or "black box"--as is common on commercial jetliners, and a cockpit voice recorder, which normally might provide clues to what happened as the aircraft climbed, probably will be of little use. The recorder, Frances said, has a 30-minute looped tape, meaning that the pilots' last words to each other probably were effectively erased at least eight times during the doomed flight.

But NTSB investigators said the possibility of an explosive decompression being the cause of the loss of radio contact with the pilots over Gainesville remains high.

Francis said such aircraft have seals on "lots of places where there are openings to outside pressure. . . . So, these are the kinds of things we'll be looking at."

Another possibility, a source close the investigation said, is that the pilot--an ex-Navy flier--failed to set automatic altitude settings that adjust pressure inside the aircraft as it climbs.

The FAA today released data on pressurization problems on all Learjet 35s. The data showed that between 1974 and 1989 there were 12 "service difficulty" reports filed involving pressurization of Learjet 35s, but only one such report after that, in 1996.

Francis said that it would be several days before the wreckage will be taken to a hangar, probably at the Aberdeen airport, and be examined for such things as seals damaged before the impact and the positions of control systems at impact.

Officials said, however, that because of the conditions of the bodies, toxicological tests on tissue probably will be of no use in determining whether the victims were incapacitated by hypoxia, or oxygen deprivation.

Aviation experts have said that when a pilot goes into a hypoxic state as a result of a loss of pressurization, he often will first experience a giddy, false sense of well-being, then lose coordination and then become unconscious in as quickly as a minute at an altitude of 30,000 feet.

Brad Randall, South Dakota's special deaths investigator, said that while tests will be conducted for carbon monoxide and drugs, among other things, the pathology for conducting such tests to determine if there was hypoxia "is not applicable in this case."

Randall said that tissue is expected to be available in a few days, and that toxicological tests for possible contributing factors other than hypoxia would take a "couple of weeks."

Francis said that recovering human remains from the wreckage was "part of the difficulty . . . of attempting to go very fast" in examining the crater.

A heavy-duty power shovel was brought to the crash site today and was expected to be used to dig a trench around the wreckage-filled crater so that investigators can examine aircraft parts without destroying evidence. About two dozen NTSB workers wearing yellow coveralls, surgical masks and gloves probed around the crater as cattle grazed nearby.

Although most of the wreckage was in relatively small pieces, Francis said, he was "amazed" to watch representatives of the plane's manufacturer "say, 'that's a horizontal stabilizer, that's an aileron.' "

Francis said NTSB investigators were traveling to Florida to talk with officials of Sunjet Aviation, the company that owned the Learjet, and that they would also interview the Air Force pilots who trailed the plane on part of its uncontrolled flight. The safety board also will conduct an investigation of all air traffic control issues and will examine the plane's maintenance records, Francis said.

One of the F-16 pilots, Capt. Chris Hamilton, said that as he pulled alongside Stewart's chartered jet, he did not see any evidence of external damage. But he said the windows were frosted over--evidence of decompression. He said he could not see inside the aircraft as he flew alongside.

In addition to Stewart, killed in the crash were the two pilots, two of Stewart's agents and Bruce Borland, 40, a golf course designer.

Walsh reported from Washington; staff writer Don Phillips also contributed to this report.

CAPTION: ONE POSSIBILITY: AIR PRESSURE LOSS (This graphic was not available)